1. Field of the Invention
The invention relates generally to optical methods and apparatus for measuring turbidity of liquids and more particularly to methods and devices for the in-situ measurement turbidity of the natural water.
2. Description of Related Arts
Turbidity sensing provides a quick, practical indication of the relative amount of suspended solids in water or suspended liquids. Many industrial and commercial bath applications can make use of integrated turbidity and conductivity sensing to improve product quality, minimize ingredient consumption, and reduce wastewater discharge. However, the inventors focused on in-situ measurement turbidity of the natural water for environmental monitoring.
U.S. Pat. No. 5,589,935 describes turbidity sensor with the capability of regulating the intensity of a light source. This sensor operates by measuring transmission light but not scattering light such that it cannot measure low turbidities of natural water of rivers, lakes and reservoirs. The turbidity sensors disclosed by U.S. Pat. No. 5,828,458 and U.S. Pat. No. 5,596,408 operate with a LED (light emitting diode) and photodiodes thereby measuring turbidity using scattering and transmission. However, they require special test cells or covers, and they cannot measure turbidity in-situ.
There are two standard specifications for turbidity that are generally in use worldwide. These are the International Standard ISO 7027 (Water quality—Determination of Turbidity, International Standard, Third Edition, 1999-12-15) and the USEPA 180.1 (Nephelometric Method 2130 B, Standard Methods for the Examination of Water and Wastewater, 1989) Both methods measure the intensity of light scattered at 90.degree to the path of incident light. The specification of the ISO standard is more stringent and requires the use of a monochromatic light source. This specification allows for greater reproducibility of a measured values and greater agreement between other measuring instruments (http://www.omega.comltechref/ph-6.html).
The turbidity sensors described in U.S. Pat. No. 5,350,922 and U.S. Pat. No. 4,841,157 use LED light sources to operate with broad range of scattering angles rather than at 90°, and their optical designs are very different from the requirements of the standard ISO 7027. The turbidity sensor described in an article by Saba Mylvaganam titled “Turbidity Sensor For Underwater Applications” has an excitation beam with a big divergence and measures the light backscattered at the average angle 145° instead of 90° as it is recommended in the standard ISO 7027 (article is available at http://www.aanderaa.com/PDF Files/Articles_TD—TN/Turbidity%20Sensor%20Desi gn%20and%20performance.pdf)
Currently, there is no portable in-situ measurement turbidity sensor for the natural water which operation complies with the standard ISO 7027 and can be freely calibrated by the user with ease and convenience.